When a solder bump is formed on a semiconductor chip, a solder is deposited on a pad and then a shape of the solder bump is changed from a mushroom shape to a hemisphere shape, followed by reflowing to join by soldering. According to a conventional soldering method, in order to form a uniform solder bump, a surface oxide film of a solder is removed with a flux to cleanse the surface of the solder bump.
However, according to a soldering process that uses a flux, small cavities (voids) may be formed in the solder bump due to decomposition of the flux. These cavities not only deteriorate electrical and mechanical properties of a formed solder joint but also destroy flatness of a semiconductor with the solder bump and may generate a harmful effect on the following semiconductor joining process. A volatile material of the decomposed flux may contaminate the inside of a reflow processor and as a result a maintenance cost may increase. In addition, a flux residue remains frequently on a semiconductor substrate to cause metal corrosion, and degradation of performance of an assembly. Further, according to a method in which the flux residue is cleansed and removed after reflow step, a time required for soldering increases because of addition of after-cleansing as a new processing step.
Therefore, as a soldering method without use of a flux, there is known a method of reducing with formic acid the solder and the joined members such as a substrate and electrode (see Patent Documents 1 to 3 and others). According to such a method of reduction, when a temperature of joined members on which the solder is mounted reaches a predetermined temperature, the solder is exposed to a reducing gas including formic acid to perform an acid treatment in which a surface oxide film is removed and are then molten to be processed.
An acid treatment by formic acid is usually performed with vaporized gaseous formic acid because a reduction start temperature of formic acid is about 150° C., and a boiling temperature of formic acid is about 100° C. At that time, to know a concentration of formic acid gas in a chamber, a method of collecting a gas in the chamber and analyzing the collected gas by gas chromatograph or a FTIR (Fourier Transform Infrared Spectrometer) may be used. However, in these cases, there are problems, for example, further provision of analyzers such as the gas chromatograph, FTIR or the like, and requirement of safety measures for collecting highly irritating formic acid in a high temperature gas state. Further, there is also a problem that the formic acid concentration cannot be measured in real time because an analysis takes time.
Further, the formic acid gas after the end of the acid treatment is forcibly discharged out of the chamber. At that time, a method of collecting the gas in the chamber and analyzing the collected gas by the gas chromatograph, the FTIR or the like to confirm whether the formic gas in the chamber is sufficiently discharged faces the same problem.
Therefore, in the acid treatment with a carboxylic acid gas such as formic acid or the like, a method of conveniently and safely measuring the carboxylic acid gas concentration in the chamber in real time is desired.